The dielectric constant of a material is a fundamental physical property of the material and is important in research and development and for the control of industrial processes. For example, careful control of the amount of water in the oil being processed is important in the formulation of margarine. Since the dielectric constants of water and oil are quite different (water 80, oil .about.3) a technique to rapidly, conveniently and accurately measure the dielectric constant of a mixture of water and oil to be processed is very useful in formulating margarine. One way of determining dielectric constant involves capacitance measuring, as the capacitance of a capacitor is proportional to a constant determined by the physical dimensions of the electrodes of the capacitor and the distance between the electrodes multiplied by the dielectric constant of the material between the electrodes. For example the capacitance, C, in farads of a parallel plate capacitor is approximated by the well known equation C =K(.epsilon.A)/d, where K is a constant, .epsilon. is the dielectric constant of the material between the plates of the capacitor, A is the area of the plates and d is the distance between the plates. In addition to the area of the electrodes and the distance between them, the "end effects" of the electrodes affect the capacitance.
One technique for determining dielectric constant by capacitance methods is shown in U.S. Pat. No. 3,025,465, assigned to the assignee of the present application. In that patent, a sample of material is placed in a test cell which consists of two plates separated by a certain fixed distance. A first set of capacitance measurements is taken, first with the test cell empty and then with the material in the cell. Then, the spacing between the plates is changed by physically moving one of the plates. A second set of capacitance measurements is taken, first with the cell empty and then with the material in the cell. The dielectric constant of the material is determined from the ratio of the difference between the capacitances measured with the material in the test cell and the difference between the capacitances measured with the test cell empty, that is, filled only with air which has a known dielectric constant.
The difference between the capacitances measured with the test cell empty can also be used to calibrate a measuring instrument. Then, the dielectric constant of an unknown material can be determined using the difference between the capacitances measured with the unknown material in the test cell in conjunction with the calibrated measuring instrument. Further details of such measuring techniques are found in U.S. Pat. No. 3,025,465.
U.S. Pat. No. 3,488,758, assigned to the assignee of the present application, also utilizes a capacitance measuring method for determining dielectric constant. However, in that patent, capacitance is not measured directly. Rather, the plates of the test cell are connected across the terminals of a free running oscillator. The capacitance across the plates of the cell serves as the capacitance of the oscillator. The frequency of the oscillator is gated to a counter so as to produce an increasing count for a predetermined time interval during which the plates are separated by a first distance, followed by a decreasing count for a predetermined interval during which the plates are separated by a second distance. As in U.S. Pat. No. 3,025,465, the distance between the plates of the test cell is changed by moving one of the plates after the first measurement. The residual count remaining on the counter is a count proportional to the change in capacitance as a result of movement of the plates. As described above, the dielectric constant of the material can then be determined using a second set of differential measurements for a material having a known dielectric constant, for example, air.
Both of the above patents require that a plate be moved in carrying out the measurement process. Movement of a plate adds time, complexity and cost to the measurement process and also increases the probability of measurement error because position measurement of the movable plate is not perfectly repeatable.